The present invention relates to micro-electro-mechanical system (MEMS) tunable filters and phase shifters.
Millimeter-wave systems have been applied in various security and sensing systems, including weather monitoring, automobile crash avoidance, and airplane landing guidance (e.g., see J. B. Mead et al., Proceedings of the IEEE, 82(12):1891-1906 (1994)). Tunable filters and phase shifters could play a key role in millimeter-wave applications, especially for multi-channel communication systems and electronically scanned antennas. Current methods for building tunable filters involve using solid-state varactors (e.g., see I. C. Hunter and J. D. Rhodes, IEEE Transactions on Microwave Theory and Techniques, vol. MMT-30(9):1354-1360 (1982)). However, there are major disadvantages to this approach, namely high losses, unacceptable signal-to-noise (SNR) ratio, and rendered linearity. Over the past decade, radio frequency micro-electro-mechanical systems (RF MEMS) provided a better alternative for building tunable filters, which are necessary for multi-band receivers. For example, MEMS varactors have been employed by some in order to realize a transmission line with voltage-variable electrical length. Tunable filters with a 3.8% tuning range at 20 GHz and a minimum insertion loss of 3.6 dB are known (Y. Liu et al., International Journal of RF and Microwave Computer-Aided Engineering, 11(5):254-260 (2001)). Entesari et al. presented wide-band tunable filters using a digital capacitor bank for 6.5˜10 GHz and 12˜18 GHz ranges with an insertion loss varying between 5.5 dB and 9 dB (e.g., see K. Entesari and G. Rebeiz, IEEE Transactions on Microwave Theory and Techniques, 53(8):2566-2571 (August 2005); K. Entesari and G. Rebeiz, IEEE Transactions of Microwave Theory and Techniques, 53(3): 1103-1110 (March 2005)). A reconfigurable low-pass filter was reported by Lee et al. using multiple contact MEMS switches. The values of the inductors and capacitors were changed independently while the filter cutoff frequency dropped from 53 GHz to 20 GHz (e.g., see S. Lee et al. IEEE Microwave and Wireless Components Letters, 14(10):691-693 (2005)). Robertson et al. presented a micromachined W-band bandpass filter at 94.7 GHz without tuning capability (e.g., see S. Robertson et al., 1995 IEEE MTT-S International Microwave Symposium Digest, 3:1543-1546 (1995)).
Techniques for building phase shifters are known. For example, ferrite materials have been utilized to change the bias field and to induce time delay of the transmitting electromagnetic wave. Other approaches include the use of solid state devices such as microwave diodes and FETs to control and manipulate the phase (e.g., see G. Rebeiz, et al. IEEE microwave magazine, 72-81, (June 2002)). While ferrite-based phase shifters consume low power, their fabrication process suffers from difficulties. Diode-based phase shifters possess advantages in their small size, their compatibility with circuit integration, and their high operational speed but typically come with high signal losses. Zuo et al. demonstrated a ferrite phase shifter with a differential phase shift of 90°/KOe.mm at a frequency of 20 GHz and an insertion loss of 0.75 dB/mm (e.g., see X. Zuo, et al. IEEE Transactions on Magnetics, 37(4): 2395-2397, (July 2001)). Shan et al. reported a 90° nonreciprocal phase shifter at 12 GHz using an H-plane ferrite-slab loaded into a rectangular waveguide (e.g., see X. Shan, et al. International Journal of RF & Microwave Computer-Aided Engineering, 13(4): 259-68, (July 2003). Glance described a 14-GHz 4-bit p-i-n microstrip phase shifter with an insertion loss of 1.4 dB with a switching time of 1 nano second and switching power of 15 mW (e.g., see Glance, IEEE Transactions on Microwave Theory and Techniques, MTT-28(6): 699-671, (June 1980)). These efforts illustrate the importance of phase shifter development in scanned radar systems. Recently, MEMS technologies have been introduced to phase shifter design and implementation. MEMS technology could potentially offer low-loss and low-power consumption to solid-state phase shifters and a common scheme is to use MEMS switches to replace the solid-state switches. Hung et al. have developed a 2-bit wide band distributed MEMS transmission line phase shifter that can have discrete phase shifts of 0°, 89.3°, 180.1°, and 272° at 81 GHz with an average insertion loss of 2.2 dB (e.g., see J. Hung, et al., 33rd European Microwave Conference, vol. 3: 983-985, Munich (2003)). Lakshminarayanan et al. presented a scheme for an electronically tunable thru-reflect-line (TRL), using a 4-bit time delay MEMS phase shifter on coplanar waveguide (CPW) sections and reported a phase shift of 90°/mm at 50 GHz (e.g., see B. Lakshminarayanan, and T. Weller, IEEE Microwave and Wireless Components Letters, 15(2): 137-139, (February 2005)).
However, nearly all the known tunable filters and phase shifter are discrete devices and lack the required resolution to continuously cover the desired band of operation. Furthermore, they suffer from high insertion loss. There is therefore a need for a MEMS-based tunable filters and phase shifters that do not suffer from the above shortcomings.